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Standard Template Library Programmer's Guide
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stack<T, Sequence>



Categories: containers, adaptors	Component type: type

Description

A stack is an adaptor that provides a restricted subset of Container functionality: it provides insertion, removal, and inspection of the element at the top of the stack. Stack is a "last in first out" (LIFO) data structure: the element at the top of a stack is the one that was most recently added. [1] Stack does not allow iteration through its elements. [2]

Stack is a container adaptor, meaning that it is implemented on top of some underlying container type. By default that underlying type is deque, but a different type may be selected explicitly.

Example

int main() {
  stack<int> S;
  S.push(8);
  S.push(7);
  S.push(4);
  assert(S.size() == 3);

  assert(S.top() == 4);
  S.pop();

  assert(S.top() == 7);
  S.pop();

  assert(S.top() == 8);
  S.pop();

  assert(S.empty());
}

Definition

Defined in the standard header stack, and in the nonstandard backward-compatibility header stack.h.

Template parameters

Parameter	Description	Default
`T`	The type of object stored in the stack.
`Sequence`	The type of the underlying container used to implement the stack.	`deque<T>`

Model of

Assignable, Default Constructible

Type requirements

T is a model of Assignable.
Sequence is a model of Back Insertion Sequence.
Sequence::value_type is the same type as T.
If operator== is used, then T is a model of Equality Comparable
If operator< is used, then T is a model of LessThan Comparable.

Public base classes

None.

Members

Member	Where defined	Description
`value_type`	`stack`	See below.
`size_type`	`stack`	See below.
`stack()`	Default Constructible	The default constructor. Creates an empty `stack`.
`stack(const stack&)`	Assignable	The copy constructor.
`stack& operator=(const stack&)`	Assignable	The assignment operator.
`bool empty() const`	`stack`	See below.
`size_type size() const`	`stack`	See below.
`value_type& top()`	`stack`	See below.
`const value_type& top() const`	`stack`	See below.
`void push(const value_type&)`	`stack`	See below.
`void pop()` [3]	`stack`	See below.
`bool operator==(const stack&, const stack&)`	`stack`	See below.
`bool operator<(const stack&, const stack&)`	`stack`	See below.

New members

These members are not defined in the Assignable and Default Constructible requirements, but are specific to stack.

Member	Description
`value_type`	The type of object stored in the `stack`. This is the same as `T` and `Sequence::value_type`.
`size_type`	An unsigned integral type. This is the same as `Sequence::size_type`.
`bool empty() const`	Returns `true` if the `stack` contains no elements, and `false` otherwise. `S.empty()` is equivalent to `S.size() == 0`.
`size_type size() const`	Returns the number of elements contained in the `stack`.
`value_type& top()`	Returns a mutable reference to the element at the top of the stack. Precondition: `empty()` is `false`.
`const value_type& top() const`	Returns a const reference to the element at the top of the stack. Precondition: `empty()` is `false`.
`void push(const value_type& x)`	Inserts `x` at the top of the stack. Postconditions: `size()` will be incremented by `1`, and `top()` will be equal to `x`.
`void pop()`	Removes the element at the top of the stack. [3] Precondition: `empty()` is `false`. Postcondition: `size()` will be decremented by `1`.
`bool operator==(const stack&, const stack&)`	Compares two stacks for equality. Two stacks are equal if they contain the same number of elements and if they are equal element-by-element. This is a global function, not a member function.
`bool operator<(const stack&, const stack&)`	Lexicographical ordering of two stacks. This is a global function, not a member function.

Notes

[1] Stacks are a standard data structure, and are discussed in all algorithm books. See, for example, section 2.2.1 of Knuth. (D. E. Knuth, The Art of Computer Programming. Volume 1: Fundamental Algorithms, second edition. Addison-Wesley, 1973.)

[2] This restriction is the only reason for stack to exist at all. Note that any Front Insertion Sequence or Back Insertion Sequence can be used as a stack; in the case of vector, for example, the stack operations are the member functions back, push_back, and pop_back. The only reason to use the container adaptor stack instead is to make it clear that you are performing only stack operations, and no other operations.

[3] One might wonder why pop() returns void, instead of value_type. That is, why must one use top() and pop() to examine and remove the top element, instead of combining the two in a single member function? In fact, there is a good reason for this design. If pop() returned the top element, it would have to return by value rather than by reference: return by reference would create a dangling pointer. Return by value, however, is inefficient: it involves at least one redundant copy constructor call. Since it is impossible for pop() to return a value in such a way as to be both efficient and correct, it is more sensible for it to return no value at all and to require clients to use top() to inspect the value at the top of the stack.